This application is based on and incorporates herein by reference Japanese Patent Application No. 2000-337833 filed on Nov. 6, 2000.
The present invention relates to a self-heating type cold-cathode discharge tube control apparatus.
A general-type cold-cathode discharge tube has, as shown in FIG. 6, an elongated tube body 1, electrodes 2 provided on both ends in the tube body 1 in its lengthwise direction, and noble gas (inert gas) 3 and mercury 4 filled in the tube body 1. This general-type cold-cathode discharge tube is actuated by application of alternating current voltage between both electrodes 2 independently of heated energy, in principle. However, the pressure of the noble gas 3 in the tube body 1 is low. Accordingly, when the cold-cathode discharge tube is actuated, if the surrounding temperature of the cold-cathode discharge tube is low, there are few opportunities of collision between electrons emitted between the electrodes 2 and gas particles 3a in the noble gas 3, and heat generation by the collision cannot be expected. As a result, the temperature of the cold-cathode discharge tube does not easily increase. The evaporation of the mercury 4 cannot be expected, and the amount of ultraviolet rays generated by collision between vapors 4a of the mercury 4 and the electrons is small. As a result, there are few opportunities of collision between a light emission layer of the inner surface of the tube body 1 and the ultraviolet rays, and the light emission luminance of the general-type cold-cathode discharge tube is low at a low temperature.
To compensate for the shortage of light emission luminance upon actuation of a general-type cold-cathode discharge tube at a low temperature, a heater is provided in the vicinity of the general-type cold-cathode discharge tube. The heater is driven by a heater drive circuit so as to increase the temperature of the tube body by heat generation by the heater, to promote evaporation of the mercury 4, to increase collision between the vapors 4a of the mercury 4 and the electrons e, to increase the light emission luminance.
In the general-type cold-cathode discharge tube, even in the case where the shortage of light emission luminance upon actuation of the cold-cathode discharge tube at a low temperature is compensated by raising the temperature of the tube body by the heater, the heater and the heater drive circuit are used as necessary component parts. That is, a control apparatus to control the general-type cold-cathode discharge tube must be provided with the heater and the heater drive circuit. As a result, the construction of the control apparatus is complicated, and further, the cost is increased.
On the other hand, the general-type cold-cathode discharge tube may be replaced by a self-heating type cold-cathode discharge tube which does not require a heater and a heater drive circuit.
The self-heating type cold-cathode discharge tube has the same construction as that of the general-type cold-cathode discharge tube except that the pressure of the noble gas in the tube body is higher than that in the general-type cold-cathode discharge tube as shown in FIG. 7. In FIG. 7, reference symbol A denotes an area of pressure and partial pressure of the noble gas in the general-type cold-cathode discharge tube, and the reference symbol C denotes an area of pressure and partial pressure of the noble gas in the self-heating type cold-cathode discharge tube.
Accordingly, if the surrounding temperature of the tube body of the self-heating type cold-cathode discharge tube is low, the mercury does not easily vaporize. However, as the pressure of the noble gas is high when the alternating current voltage is applied between the electrodes, the gas particles of the noble gas and the electrons more easily collide with each other than in the general-type cold-cathode discharge tube. Thus the temperature rises due to the heat generation by the collision. Accordingly, the mercury can more easily vaporize than in the general-type cold-cathode discharge tube.
Therefore, it is proposed to improve the shortage of light emission luminance at a low temperature by raising the temperature of the tube body by boosting the flow of electrons as a current, emitted between the electrodes.
However, if the same current boosting is performed at a high temperature in the self-temperature-rise type cold-cathode discharge tube, as the pressure of the noble gas is high, the temperature of the tube body tends to rise excessively, and the life of the cold-cathode discharge tube is shortened.
To address this inconvenience, it is necessary to always monitor the temperature of the tube body of the cold-cathode discharge tube and to control in real time the current which flows through the tube body in correspondence with the temperature of the tube body so as to limit the temperature from rising excessively. As a result, however, the control of the self-heating type cold-cathode discharge tube becomes complicated.
The present invention has been made to solve the above problems and has its object to provide a control apparatus which improves a luminance rise of a self-heating type cold-cathode discharge tube at a low temperature with a simple control without controlling excessive temperature rise of the discharge tube.
In accordance with the present invention, a control apparatus uses characteristic data, previously determined such that boosting time to boost a current which flows through a cold-cathode discharge tube upon actuation of the tube becomes shorter with a rise of detected temperature of the cold-cathode discharge tube and becomes longer with a fall of the detected temperature.
When the detected temperature is in a predetermined low temperature range as a cause of shortage of luminance, the control apparatus determines the boosting time in correspondence with the detected temperature. During the boosting time, the control apparatus controls the cold-cathode discharge tube with an alternating current voltage determined to properly maintain the light emission luminance of the discharge tube. In this arrangement, even if the temperature of the cold-cathode discharge tube is low, the cold-cathode discharge tube performs excellent light emission without shortage of luminance. Further, as this advantage can be attained by utilizing the boosting time determined based on the characteristic data, the control process does not become complicated.